Electromagnetic force motor having adjustable magnetic saturation

ABSTRACT

An improvement in electromagnetic force motors in which the force versus input signal characteristic is adjustable by providing in the flux path regions of magnetically saturable material defining magnetic saturation paths and members of magnetically permeable material insertable in the regions to decrease the lengths or cross-sectional areas of the saturation paths.

United States Patent [72] Inventor John B. Pegram [56] References CitedW Alleles, Calif. UNITED STATES PATENTS gm- 3 2,488,443 11/1949Sonnemann..... 33s/279x if P t d 1,906,027 .4/1933 Wahl 336/132X 1 9''5:2 c 2,594,088 4/1952 Sonnemann m1. 335/230 [731 Awgn" 3,434,033 3/1969Lev/is 335/230 Primary ExaminerG. Harris Attorney- Nilsson & Robbins[54] ELECTROMAGNETIC FORCE MOTOR HAVING ADJUSTABLE MAGNETIC SATURATIONABS-TRACT A" m provement 1n electromagnehc force mos cm 20 Dnwmg Figstors in which the force versus input signal characteristic is ad- 1 [52]US. 335/230, justable by providing in the flux path regions ofmagnetically 335/227, 335/279 saturable material defining magneticsaturation paths and [51] Int. Cl H0ll'7/l3 members of magneticallypermeable material insertable in the [50] Field of Search 336/132,regions to decrease the lengths or cross-sectional areas of thesaturation paths.

ELEEIROMAGNETIQ FORCE MOTOR HAVING AIMIJSTAFALE MAGNETIC SATURATIONBACKGROUND OF THE INVENTION 1. Field of the Invention The fields of artto which the invention pertains include the fields of torque motors,fluid handling, valve and valve actuation.

2. Description of the Prior Art The use of force motors in the prior arthas long been known, One example of the utility of force motors is inthe positioning of the pilot valve such as a flapper or spool of ahydraulic servovalve. The force motor in such applications is used as anelement of a control system and is adapted by its operatingcharacteristics to convert an electrical input signal into a desiredmechanical position of the spool valve. Force motors also have a widevariety of other uses, for example, in

automatic control systems for industrial machinery.

Such force motors typically utilize a large percentage of the air gapfor armature motion. In various of the prior art applications it haslong been desirable to provide force motors which bear a linearrelationship to the applied input signal and wherein the force exertedupon the armature remains substantially linear as it approaches the polefaces. This would permit displacement of the armature over the entiregap.

A variety of schemes have been proposed to accomplish suchlinearization, for example, reducing the effective length or thecross-sectional area of a part of the armature and keeping that portionalways saturated magnetically, as illustrated in US. Pat. No. 3,071,714.Another linearization scheme provides recesses on the face of thearmature and a plurality of mating teeth on the face of the pole pieces,for example, as shown in US. Pat. No. 2,930,945. A more recent andimproved method of linearization is described in application Ser. No.565,237, filed Jul. 14, 1966 by Samuel A. Gray, entitled ElectromagneticForce Motor ll-iaving Linear Output Characteristics," now US. Pat. No.3,517,360.

Each of the foregoing devices utilizes magnetic saturation to obtainlinearization. However, problems arise in manufacturing such torquemotors. Variations in effective pole areas, magnetic saturationcharacteristics, leakage path variations, magnetic strength variations,hysteresis curve characteristics, normal production tolerances for airgaps, and the like, make the manufacture of acceptable devices verycostly. There is a need, therefore, for a simple method of assuring theattainment of magnetic saturation appropriate to the linearization ofthe force versus input signal characteristic of the force motor.

SUMMARY OF THE lNVENTlON The present invention enables the manufactureon a production basis of electromagnetic force motors in which themagnetic saturation of each motor is adjustable so as to iinearize theforce versus input signal characteristic of the motor and compensate forall manufacturing tolerances. The present invention provides meanswhereby the length and/or cross-sectional area of the saturation pathmay be adjusted during a test of the motor to appropriately modify theforce versus input signal characteristic of each motor. A motor isprovided of the electromagnetic force type, that is, having a pair ofpole pieces spaced apart to provide an air gap therebetween, an armaturewithin the air gap and permanent magnetic means defining a magneticcircuit. The improvement of this invention comprises providing at leastone region of magnetically saturable material in the flux path of themagnetic circuit and providing a discrete member of magneticallypermeable material for association therewith. The region of magneticallysaturable material is constructed to define a magnetic saturation pathand to define means for receiving the discrete member thereat to therebydecrease the length or cross-sectional area of the saturation path. At aselected armature position polarizing flux from the permanent magnet andsignal coil is sufticient to magnetically saturate the region in theabsence of the discrete member, but is insufficient to magneticallysaturate portions of the region that are occupied by the discretemember.

in particular embodiments, a plurality of magnetically saturable regionsis provided and a plurality of discrete members therefor. Withappropriate adjustment, the magnetically saturable regions and discretemembers define a plurality of discrete sections, each section beingmagnetically saturable at a predetermined level of magnetic fluxdensity. The motor can be constructed so that the level of magnetic fluxdensity at which the sections saturate is different for each section.The result is a linearization of force with input signal as each sectionis successively saturated.

' The saturation-adjustable regions can be positioned within thearmature of the motor, within one or both of the pole pieces, betweenthe permanent magnets and the motor frame; i.e., any convenientlocations.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a side elevational view ofan electromagnetic force motor utilizing the present invention;

FIG. 2 is a top elevational view of the apparatus illustrated in FIG.ll;

FIG. 3 is a cross-sectional view taken about the line 3-3 of FIG. 2, inthe direction of the arrows;

FIGS. 4-9 are plan and cross-sectional views, of a variety of particularmagnetically saturable sections and insertable members therefor;

FIG. 10 is a top elevational view of an electromagnetic force motorconstructed in accordance with another embodiment of the presentinvention;

FIG. II is an enlarged view of a portion of the apparatus illustrated inFIG. 10;

FIG. 12 is a view similar to that of FIG. 11 but of still anotherembodiment of this invention;

F1613 and 14 are plan views of insertable members utilizable in theapparatus of FIG. II or 112;

FIG. 15 is a top elevational view of an electromagnetic force motorutilizing still another embodiment of this invention;

FIG. 16 is a cross-sectional view taken about the line lib-I6 of FIG.15, in the direction of the arrows;

FIG. 17 is an enlarged view of a portion of the apparatus illustrated inFIGS. 15 and 16;

FIG. 18 is a view similar to that of FIG. 17 but utilizing anotherembodiment of this invention; and

FIGS. 19 and 20 are curves illustrating operating characteristics of anelectromagnetic force motor constructed in accordance with the presentinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIGS. 1 through 3,there is illustrated an electromagnetic force motor constructed inaccordance with one embodiment of the present invention. The force motorincludes frame members I0 and ii having a pair of permanent magnets l2;and i3 affixed thereto in any manner or means known to the art such, forexample, as by brazing. At the upper surface of the frame members l0 and11 there is affixed a pair of pole pieces lid and 15. The pole pieces 14and I5 are adjustable by way of loosening attaching bolts to and I7therethrough and adjusting the position of the pole pieces 1d and 15until they are of the desired relationship to an armature 18 disposed inthe air gap formed between the faces of the pole pieces I4 and 15. Thearmature 18 is supported upon a support member 20 and may have anydesired structure such, for example, as an elongated tube which flexesabout a pivot point P to thereby position a flapper 21 which is used tocontrol the position of a servovalve spool as above referred to. Themounting means 20 may be sealed by means of a conventional O-ring' 22 tothe base 23 as is well-known to the art. Positioned about the armatureit; is a coil 25 which is adapted to receive an electrical input signaland which thereby causes the armature to take a physical positiondepending upon the strength of the applied input signal and the polaritythereof.

for the receipt of mating plugs 32, 33, 34 and 35 of magneticallysaturable material, which plugs are threadable into the cavities bymeans of drive slots 36, 37, 33 and 39, respectively, therein. The plugs32-35 can be of any appropriate magnetically permeable material of thesame, greater or less permeability or saturation level as thesurrounding material of the inserts 26-25.

As part of the manufacture procedure, the magnetically saturable plugs36, 37, 38 and 39 are threaded into their respective cavities untilcompletely inserted therein. The force versus input signalcharacteristic is then plotted by methods well-known to the art and willtypically have an asymptotic curvature with respect to the force axis,until pole contact, representing rapidly increasing force with armaturemovement. The permanent magnets 12 and 13 provide sufficient polarizingflux to magnetically saturate those regions of the inserts 26-23adjacent the cavities in the absence of the insert plugs 32-33, but themagnets provide insufficient polarizing flux to magnetically saturateportions of the inserts 26-29 occupied by the plugs 32-35, Although theintensity of the flux increases somewhat as the reluctance of themagnetic circuit decreases, this increase is insufficient to offset therelatively much larger increase in length or area of the flux path.Accordingly, when the plugs 32-35 are fully inserted, the inserts 26-29are not magnetically saturated. The plugs 32-35 are then withdrawn untilthe force versus input signal obtains an S-shape, e.g., by uniformlywithdrawing the plugs 32-35, partially from their cavities. The plugs32-35 can then be individually adjusted so as to straighten out theS-shaping and Iinearize the force versus input signal characteristic.The basis of such adjustment will be discussed further with respect toFIGS. ll) and 2t and lies in providing varying lengths of unoccupiedcavities in the inserts 26-29. Thus, discrete regions of varyingcross-sectional areas are provided. As the flux density increases uponmovement of the armature 18 toward one or the other of the poles M or15, that discrete region having the smallest cross-sectional area isfirst driven into saturation. Each of the regions of magneticallypermeable material that is progressively greater in cross-sectional areais then sequentially magnetically saturated to produce a stair-step typerelationship between flux increase and armature displacement. By properadjustment of the length of the cavities, a substantially linear forceversus input signal curve can be obtained.

in place of the cylindrical plugs 32-35, one can utilize plugs of avariety of configurations, as illustrated in FIGS. 4-- 9. in each of theH65. 4-9, the direction of the flux path is shown by the arrows 36.FIGS. Ala and ib show plan and crosssectional views, respectively, of aninsert 38 and plug 40 of square cross-sectional configuration that maybe substituted for any of the inserts 26-23 or plugs 32-35 of FIG. 2. Agenerally longer length of saturation path will thereby be obtained. Theplug iii is pressed into the insert 38 and can be forcefully slid to africtionally retained position. The same arrangement can be utilized forthe plugs 32-35, rather than the screw-fit described above; or any otherretention means can be utilized which allows the plugs to be raised orlowered as described.

Another alternate configuration is shown in plan and crosssectionalviews in FIGS. a and 5b, respectively, wherein the insert 42 is formedto accommodate a trapezoidal-shaped plug 44. A generally longersaturation path is thus provided than with the plugs 32-35 of FIG. 2.The shaping of the plug as imparts a particular unique shape to theforce versus input signal characteristic.

Alternatively, as shown in plan and cross-sectional views in FIGS. 6aand 6b, respectively, one may provide an insert 46 formed to accommodatea plurality of strip-shaped plugs 43, Silt and 52 which can beindependently raised or lowered to obtain very long saturation pathswith a variety of unique force versus input signal characteristics.

FIGS. 7a and 7b shown plan and cross-sectional views, respectively, ofan insert 54 formed to receive a plug 56 of triangular cross section toprovide a still different force versus input signal characteristic.

FIGS. 8a and 8b show plan and cross-sectional views, respectively, of aninsert 58 formed to receive a rotatable vanelike plug 60. FIGS. 8c and8d show cross-sectional views of the insert 58 and vanelike plug as inalternative rotational positions. The saturation area can be varied byrotating the plug so and/or lowering or raising the plug on with respectto the insert 58. I

FIGS. 9a and 9b show plan and cross-sectional views, respectively, of aninsert 62 formed to accommodate a slotted plug 6d therein. FIGS. 9c and9d show alternative positions of the slotted plug 64 in the insert 62.With the plug 64 positioned as shown in FIG. 9c a short saturation pathis provided of relatively small cross-sectional area. In the positionshown in FIG. 3a, a relatively long saturation path is provided with arelatively large cross-sectional area. The cross-sectional area of thesaturation path can be varied by raising or lowering the slotted plugs64 and both the cross-sectional area and the length of the saturationpath can be varied by rotating the plug 64.

Referring to FIGS. it) and 11, still another alternative embodiment ofthis invention is illustrated. An electromagnetic force motor isconstructed that is similar to the motor of F lGS. 1-3 but in which thepermanent magnets 66 and 67 directly abut frame members (not shown) thatsupport poles 63 and 69, by means of attaching bolts 70 and 71, todefine an air gap with an armature 72 therein.

In the embodiment of FIGS. 1-3 plugs 26-29 were provided between thepermanent magnets 12 and i3 and the frame members it) and ii. Incontrast, in the embodiment of FIGS. 10 and ii, adjustable plugs 73 and74 are provided on the armature 72. The plugs 73 and 7d are shownthreadable in and out of pockets 73 and 76, respectively, therefor butmay be formed to be threadable therein. By threading the plugs 73 and 74in and out of the pockets 75 and 76 therefor, one can vary thecross-sectional area of the saturation path in the armature to achievedesired force versus input signal characteristics.

FIG. 12 depicts an embodiment wherein an armature 77 is positionedbetween poles 753 and 79 therefor in a manner similar to the embodimentof FIG. 11 but wherein pockets tit and 81 and plugs 82 and 83,respectively, therefor are uniquely shaped to obtain particular forceversus input signal characteristics. The plugs 32 and 83 are shaped sothat different lengths of saturation paths are encountered therealong.FIGS. 13 and 114 illustrate still other alternative configurations forplugs 84 and 85 that can be substituted for the plugs 32 and/or 83 toobtain other unique force versus input signal characteristics. Note,that the plug 85 depicted in FIG. 14 is shaped to provide ageometrically decreasing saturation path length during insertionthereof. Each plug of FIGS. 12, i3 and M are merely inserted in arelatively easy fitting pocket and seated in. Fressfit plugs could beutilized, but it should be noted that it is generally undesirable to usepressfit plugs in the armature in view of the resultant widening of thehysteresis loop upon the consequent hardening of the armature.

Referring to FIGS. 15 through 17, there is illustrated anelectromagnetic force motor constructed in accordance with still anotherembodiment of the present invention. Here too, the force motor includesframe members 36 and 87 having a pair of permanent magnets 88 and 83,but they directly abut the frame members as and 87. At the upper surfaceof the frame members 86 and 87 there is affixed a pair of pole pieces 93and 91 which are adjustable, by means of loosening attaching bolts 92and 93, to the desired relationship to an armature Mi disposed in theair gap formed between the faces of the pole pieces 33 and 9t. Here too,the armature M is supported upon a support member 95 about a pivot pointF to thereby position a flapper 5.

The pole faces of the pole pieces 9% and 91 are arranged to provide asubstantially linear force versus signal current characteristic to theelectromagnetic force motor. With particular reference to Phil. l7, eachpole E li and @i has provided therein a plurality of discrete threadableplugs 97 received in threaded pockets 98 (HG. l6) therefor. Each plug 97is provided with a drive slot 99 and is threadable into its pocket 9%,each independently of the other, as shown more clearly in Fill in. Sinceeach of the discrete regions defined by the particular plug 9'7 andpocket 9&3 therefor has a different crosssectional area, the regionswill be driven into saturation at dif- -ferent points in time by thepolarizing flux provided by the permanent magnets 3b and 89 signal coil.in this particular embodiment, the plugs 97 are disposed so that thepolarizing flux is not sufficient to drive any of. the discrete regionsdefined by the plugs 97 into magnetic saturation when the armature lhtiis centered. However, upon the application of an electrical signal tothe coils, for example, each as to polarize the upper portion of thearmature wt as a south-magnetic pole, and assuming that the pole piece9b is magnetized to be a north-magnetic pole, then the armature llllilmoves toward the pole piece 9i). As this movement occurs, the fluxdensity between the armature W and the pole piece 90 increases. As theflux density increases, the magnetically saturable material in thediscrete plug 97 regions sequentially become saturated in apredetermined manner. Thus, as the flux density begins to increase, thatdiscrete region having the smallest crosssectional area of magneticallypermeable material is the first driven into saturation and then each ofthe regions of progressively greater cross-sectional area goessequentially into magnetic saturation. in terms of the plugs 97, as theflux density begins to decrease, those pockets Wt having the leastpenetration by its plug 97 is first driven into saturation and then eachof the pockets 98 having progressively greater penetration by their plug97 goes sequentially into magnetic saturation.

By such means, discrete regions of varying cross-sectional areas can beprovided and with appropriate stepping, as indicated in application Ser.No. 565,237, referred to above, stepwise linearization of the forceversus input signal characteristic can be obtained. The great advantageof this invention is that the saturation sequence can be very accuratelycontrolled. A plurality of determinations of force versus input signalcharacteristics can be coordinated with individualized adjustment of thedepth of penetration of the plugs 97 into the pockets 9%. The resultantsequence is illustrated in HG. 19. Upon the application of the currentsignal the flux density present in the pole piece 9%? increases as thearmature Mill approaches its pole face. As the flux density approachesthe first arbitrary point such as that shown at Mill, a discrete region,for example at 97a, has reached saturation and therefore no longer hasany capability of increasing the pulling force on the armature Mill. Asthe armature, however, continues to approach the pole face of the pole90, a second flux level such as that shown at W2 is reached at which,for example, the region at plug @717 goes into magnetic saturation. Thisproduces a stair-step type curve which, however, by the proper selectionand adjustment of the plugs 97 is substantially linear as is illustratedin l9.

As an additional manner of illustrating the principal upon which thisembodiment is based, reference is made to HG. 24) which is agraphillustrating force and magnetic saturation on the ordinate andinput signal current on the abscissa. Curve A represents the typicalforce curve of prior art electromagnetic force motors showing thenonlinear relationship of the force to the input signal current. Curve Bon the other hand represents the overall magnetic saturation of thevarious sections of the pole 9b superimposed over the curve A. By addingthese two curves A and B one then obtains curve C shown in dashed linewhich is a substantially linear force versus signal currentrelationship.

The various discrete adjustable regions of magnetically saturablematerial need not necessarily be disposed in the poles as shown in F168.-17 but may be disposed in the armature, and this is illustrated in H0.38. As is therein shown, the armature W3 is positioned between the polefaces of a pair of oppositely disposed poles W5 and 199. However, inthis embodiment, the armature N33 has disposed therein a plurality ofthreaded pockets and plugs Mid therefor of magnetically saturablematerial, each being individually adjustable to penetrate into itspocket a different length to provide different cross-sectional areastaken normal to the lines of magnetic flux so that each of the regionsthereat saturate at a different point as above described.

In each of the alternative embodiments of an electromagnetic force motorconstructed in accordance with the present invention it should berecognized that the magnetic circuit has inserted therein an adjustabledynamically varying reluctance the value of which depends upon the fluxdensity present in the magnetic circuit. The polarizing flux from thepermanent magnets and signal coil is sufiicient to magnetically saturatethe plug regions at least at some selected armature position within theair gap and the absence of the plug therein, but is insufficient tomagnetically saturate portions of the regions occupied by the plug atthat armature position. By utilizing structures of the presentinvention, the fringing flux normally present at the force motor gapschanges much less with the position of the armature than with ordinarytorque motors. Thus, the overall efficiency and force-output of anelectrical force motor in accordance with the present invention issubstantially increased.

lclaim: l. In an electromagnetic force motor having a pair of polepieces spaced apart to provide an air gap therebetween;

an armature positioned with a portion thereof in said air gap, saidarmature being adapted to move in said air gap from a neutral positionthereby more closely approaching one of said pole pieces; a magneticcircuit in said motor including said pole pieces,

said armature and a permanent magnet means, said permanent magnet meansproviding polarizing flux; electrical signal receiving means operativelyassociated with said magnetic circuit to establish magnetic flux thereinof a strength proportional to a received electrical signal; theimprovement of apparatus for enabling adjustment of force versus inputsignal characteristic, comprising: and at least one region ofmagnetically saturable material disposed in the flux path of saidmagnetic circuit, said region defining a magnetic saturation paththereat;

a discrete member of magnetically permeable material;

said region defining means for receiving said discrete member thereat tothereby vary the length or cross-sectional area of said saturation path;

said permanent magnet polarizing flux and signal receiving meansmagnetic flux being sufficient to magnetically saturate said region at aselected armature position within said air gap in the absence of saiddiscrete member, but being insufficient to magnetically saturate thoseportions of said region that are occupied by said discrete member atthat armature position.

2. An electromagnetic force motor as defined in claim l in which saidmeans for receiving said discrete member is a cavity defined by saidregion, said discrete member being insertable in said cavity.

3. An electromagnetic force motor as defined in claim 1 in which saidmagnetically saturable region is positioned within said armature.

3. An electromagnetic force motor as defined in claim l in which saidmagnetically saturable region is positioned within one of said polepieces.

5. An electromagnetic force motor as defined in claim l which furtherincludes a frame member securing said pole pieces in operative.association with said permanent magnet means, said magneticallysaturable region being disposed in said magnetic circuit between saidframe member and said permanent magnet means.

An electromagnetic force motor as defined in claim 2. wherein there is aplurality of said magnetically saturable regions and a plurality of saiddiscrete members of magnetically being magnetically saturable each at adifferent level of flux density.

d. An electromagnetic force motor as defined in claim 7 wherein each ofsaid plurality of discrete members is receivable independently of theothers of said plurality of discrete members.

1. In an electromagnetic force motor having a pair of pole pieces spacedapart to provide an air gap therebetween; an armature positioned with aportion thereof in said air gap, said armature being adapted to move insaid air gap from a neutral position thereby more closely approachingone of said pole pieces; a magnetic circuit in said motor including saidpole pieces, said armature and a permanent magnet meanS, said permanentmagnet means providing polarizing flux; electrical signal receivingmeans operatively associated with said magnetic circuit to establishmagnetic flux therein of a strength proportional to a receivedelectrical signal; the improvement of apparatus for enabling adjustmentof force versus input signal characteristic, comprising: and at leastone region of magnetically saturable material disposed in the flux pathof said magnetic circuit, said region defining a magnetic saturationpath thereat; a discrete member of magnetically permeable material; saidregion defining means for receiving said discrete member thereat tothereby vary the length or cross-sectional area of said saturation path;said permanent magnet polarizing flux and signal receiving meansmagnetic flux being sufficient to magnetically saturate said region at aselected armature position within said air gap in the absence of saiddiscrete member, but being insufficient to magnetically saturate thoseportions of said region that are occupied by said discrete member atthat armature position.
 2. An electromagnetic force motor as defined inclaim 1 in which said means for receiving said discrete member is acavity defined by said region, said discrete member being insertable insaid cavity.
 3. An electromagnetic force motor as defined in claim 1 inwhich said magnetically saturable region is positioned within saidarmature.
 4. An electromagnetic force motor as defined in claim 1 inwhich said magnetically saturable region is positioned within one ofsaid pole pieces.
 5. An electromagnetic force motor as defined in claim1 which further includes a frame member securing said pole pieces inoperative association with said permanent magnet means, saidmagnetically saturable region being disposed in said magnetic circuitbetween said frame member and said permanent magnet means.
 6. Anelectromagnetic force motor as defined in claim 1 wherein there is aplurality of said magnetically saturable regions and a plurality of saiddiscrete members of magnetically permeable material each independentlyreceivable at respective ones of said regions.
 7. An electromagneticforce motor as defined in claim 1 wherein there is a plurality of saidmagnetically saturable regions and a plurality of said magneticallysaturable regions and a plurality of said discrete members ofmagnetically permeable material receivable at respective ones of saidregions to thereby provide a plurality of discrete sections, saidsections being magnetically saturable each at a different level of fluxdensity.
 8. An electromagnetic force motor as defined in claim 7 whereineach of said plurality of discrete members is receivable independentlyof the others of said plurality of discrete members.